The aim is to learn how basement membranes are made during development, so as to lay a foundation for understanding their lesions and repair in blood vessels and other tissues. A principal component of basement membranes is procollagen IV. We have followed its synthesis by endothelial and teratocarcinoma cells, located its specialized carboxyl end, and traced its assembly to a supramolecular net via an intermediate of four molecules joined through their amino ends, which we characterized. We shall determine how the carboxyl ends come to join pairwise in cell cultures and in tissues, and whether there are additional junctions between procollagen molecules. We shall find the connections of proteoglycan, laminin, entactin and other components to the collagen scaffold. We will relate basement membrane assembly to cell division and tissue growth by studying its formation in embryo organs. Basement membranes occur universally and are formed during development of cell sheets. We characterized a procollagen similar to procollagen IV in larvae and cell cultures of Drosophila, and have a genomic DNA clone for it. We shall further characterize the protein and its supramolecular associations. We shall follow its synthesis, and of its m-RNA, at successive developmental stages in normal organisms and in a conditional, embryonic lethal mutant in basement membrane assembly. We have isolated the basement membrane laminin, of unique electron microscopic appearance, from Drosophila. We made antibodies to it, and also to the procollagen, and to a proteoglycan. All these stain basement membranes. We shall determine when these and other materials are made during development and how they form basement membranes. We shall also study this in primary cell cultures of dispersed embryos and see how cell differentiation is related to the synthesis of basement membrane materials. We conclude that there is a strong, common theme to the structure of basement membranes. Our parallel studies of vertebrate and invertebrate basement membranes, and of a conditional mutation, will help to uncover assembly steps that are prone to partial disruption. They will be examples of potential lesions of human basement membranes.